The present invention relates to a magnetic bubble memory device having propagation tracks formed by ion implantation.
FIG. 1 illustrates the principle of the magnetic bubble propagation in a magnetic bubble memory device having propagation tracks formed by ion implanation. A magnetic layer 2 having a uniaxial anisotropy and capable of holding magnetic bubbles 4 is formed on a non-magnetic substrate 1. A contiguous-disk mask made of, for example, molybdenum and silicon is provided on the magnetic layer 2 and an unmasked region is implanted with hydrogen ions or neon ions to form an ion-implanted layer 3. In this structure, the magnetization 8 of the ion-impanted layer 3 lies in a plane of the layer 3. The direction of the magnetization 8 parallel to the plane of the ion-implanted layer 3 can be changed by a rotating magnetic field 6 applied in parallel to the plane of the layer 3. At a boundary between the ion-implanted region and the non-implanted region 5 are developed charged walls 9 each having a magnetic pole due to convergence or divergence of the magnetization 8. When the charged wall 9 is caused to move along the boundary, i.e. the edge of the bubble propagation track under action of the in-plane rotating field 6, the magnetic bubble is propagated following the movement of the charged wall 9.
Bubble propagation characteristics have been examined when the width w (see FIG. 2) of the non-implanted region as measured from the bottom 11 of a cusp 10 located along the propagation track having a periodical length of 4 .mu.m was varied. In this connection, the term "cusp" refers to a concaved area which is located within the region implanted with ions to form the meandering propagation track and is enclosed by the boundary between the ion-implanted region and the non-implanted region at a smaller angle than 180.degree..
FIG. 3 graphically illustrates relationships between the biasing magnetic field and the rotating magnetic field with the width w being taken as a parameter. The abscissa represents the intensity of the rotating field and the ordinate represents the intensity of the binasing field under which the bubble can be propagated. It will be seen from FIG. 3 that the minimum value of the rotating field becomes greater as the width w of the non-implanted region is increased. FIG. 4 graphically shows the results of examination of the charged walls appearing along the propagation track, the examination having been carried out by Bitter technique (using "Ferrofluid"). More specifically, it has been examined in which phase of the rotating field appears a charged wall 9 (FIG. 5) necessary for driving a magnetic bubble at the bottom of a cusp into the next adjacent cusp. In FIG. 4, the phase or angle of the rotating field at which the charged wall 9 appears with reference to the direction of 0.degree. (degree) shown in FIG. 5, is taken along the ordinate. The width w of the non-implanated region measured from the bottom 11 of the cusp is taken along the abscissa. A curve a corresponds to the case of the rotating field of 30 Oe, while a curve b corresponds to the case of the rotating field of 60 Oe. As will be seen from FIG. 4, the charged wall 9 appears with greater phase delay as the width w of the non-implanted region is increased. Reference may be made to Kodama et al: Digests of the fifth annual conference on Magnetics of the Magnetic Society of Japan, October 1981, p. 164.
In the hitherto known magnetic bubble device in which magnetic bubbles of 2 .mu.m in diameter are employed, a structure implementing minor loops and two major lines has been adopted. More specifically as shown in FIG. 6, the magnetic bubble memory device comprises a group of minor loops 13 for storing information, a write-in major line 16 including a generator 14 and swap gates 15 for writing information, and a read-out major line 19 including replicate gates 18 each for dividing the bubble of the associated minor loop 13 into two upon reading of information to transfer one thereof to a detector 17. When a structure similar to such a magnetic bubble memory device employing magnetic bubbles of 2 .mu.m is to be applied to a high density magnetic bubble memory device using magnetic bubble of 1 .mu.m in diameter, it is required to reduce not only the periodicity of the basic propagation track in each minor loop but also the periodicity of the minor loops. To meet this requirement, functional parts such as the swap gates, the replicate gates and other have to be reduced in size, which means that the width of conductors provided in association with the functional parts must be correspondingly decreased. However, since the conductors are usually formed of a metal such as Au, Al-Cu or the like, migration will take place when the current density is excessively high. In other words, an upper limit is imposed on magnitude of the current density, giving rise to a problem that the current required for controlling the magnetic bubbles can no more be conducted. To evade this difficulty, there has been proposed a structure in which the periodicity of the minor loops is increased by folding each of them twice or more in a manner shown in FIG. 7, thereby enlarging the areas occupied by gates (see S. Orihara et al: "An 8 .mu.m Period Bubble Memory Device with Relaxed Function Designs", IEEE TRANS., Vol. MAG-15, 1979, p. 1692 and also see M. Y. Dimyan and W. C. Hubbell: "Design and Operation of a 550K bit enhanced density 3 .mu. m bubble memory chip", J. Appl. Phys. Vol. 50, 1979, p. 2225).
When the minor loop propagation track of the folded type mentioned above is made by ion implantation, there will be involved a propagation track pattern which is accompanied with a non-implanted region 20 of a large area as shown in FIG. 8 which shows in an enlarged view an area enclosed by a broken line circle A in FIG. 7. Examination of this region 20 with respect to the bubble propagation characteristic has shown that the magnetic bubble propagation cannot be accomplished in portions 21, 22 and 23 shown in FIG. 8 though the bubbles are propagated in the other portions.